Current sensors are required in electrical apparatus to sense if excessive current is being drawn by the apparatus. Currents that exceed the tolerance of circuit elements within the apparatus can cause these elements to overheat and destruct. This danger is especially significant for etched circuit boards, where overheating and destruction of elements soldered to the board can result in costly and time-consuming repair. In many cases it is less expensive merely to replace the entire board than to repair the damaged elements.
In the prior art, both indirect and direct methods of current sensing are used to detect excessive currents within an etched circuit board. A common indirect technique is to mount a fuse in series with the power supply line. The fuse is rated at a predetermined amperage so that it will melt if the apparatus begins to draw excessive current. However, fuses are notoriously inaccurate, often having an accuracy of 100%; that is, a fuse with a 10 amp rating may not fail until 20 amps are drawn through it by the circuit. Circuit elements with a current tolerance just above the nominal rating of a fuse are thus not adequately protected.
A more direct technique is to mount a current sensing resistor in series with a current path on an etched circuit board and then measure the voltage drop across the known resistor to determine the current drawn. This technique is much more accurate than a fuse, but it also suffers from several drawbacks. First, the accuracy of conventional resistors is relatively poor in the milliohm range. A current sensing circuit should ideally work in the millivolt and milliohm range so that the power consumed by the sensing circuit is small. For example, a 10 amp current through an etched circuit board can be measured with a 30 millivolt drop across a 3 milliohm resistor. Secondly, the extra resistor is typically soldered onto the circuit board, and leads are soldered from the ends of the resistor to the rest of the sensing circuit. This soldering introduces unknown resistances into the sensing circuit, resistances which can easily exceed the value of the resistor and thus distort the current measurement. These errors may be corrected somewhat by use of a four-terminal Kelvin resistor with the sensing circuit leads integral within the resistor. However, such resistors are expensive and not accurate in the milliohm range. Moreover, apart from accuracy, the use of a Kelvin resistor still introduces considerable unknown resistances into the sensing circuit because of the soldering points connecting it to the circuit board.
The present invention meets these drawbacks by providing an accurate, inexpensive means for sensing current within an etched circuit board and signaling if such current exceeds a predetermined level.